PMP8740

anjana salgaonkar

Other Parts Discussed in Thread: PMP8740, UCC28950

Dear Roberto,

I am stucked at output capacitor as per the formula given in SLUA560C it gives value greater than selected one. Please explain how do you calculate output capacitor value?

Anjana

over 5 years ago

0 Roberto S. over 5 years ago

TI__Expert 5225 points

Hi Anjana,

The output capacitor, calculated in the excel sheet, has the same calculation procedure as in the SLUA560C: it's made by setting the maximum deviation during a transient response (in my case 5% of Vout, 32V, which is 1.6V). The result is 1936uF. I selected in the excel sheet 4 x 470uF, which are barely sufficient. In the final prototype I selected 6 x 470uF to have more margin.

Regards,

Roberto

0 anjana salgaonkar over 5 years ago in reply to Roberto S.

Expert 1460 points

do you mean Vtran =1.6v in your case.

0 anjana salgaonkar over 5 years ago in reply to anjana salgaonkar

Expert 1460 points

Also please check , am I considering correct specifications as attached

0 Roberto S. over 5 years ago in reply to anjana salgaonkar

TI__Expert 5225 points

Hi Anjana,
Yes, I meant VTRAN = 1.6V. Can you please attach the excel sheet you are working on?
Thanks,
Roberto

0 anjana salgaonkar over 5 years ago in reply to Roberto S.

Expert 1460 points

DEAR ROBERTO ,

ATTACHING EXCEL SHEET FOR PMP8740 DESIGN. PLEASE CHECK AND SUGGEST CORRECTIONS.

THANKS,

ANJANAUCC28950 _PMP8740.xlsx

0 Roberto S. over 5 years ago in reply to anjana salgaonkar

TI__Expert 5225 points

Dear Anjana,

I see that in your excel sheet the time "Thu" has a 10^-6 factor, which should not be there, because the normal calculation takes into account already that the result is in usec and not seconds. If you correct the formula in the cell, the it should give ~ 13usec and the output capacitance several thousand uF. This big capacitor comes from the fact that you set a small voltage deviation during output load transients (VTRAN) compared to my design, and also because you are switching at 50KHz (on the bridge) and 100KHz (on the output inductor), which is "fs", cell C18.

Please let me know if you get reasonable values.

Best regards,

Roberto

0 anjana salgaonkar over 5 years ago in reply to Roberto S.

Expert 1460 points

Thank a lot dear Roberto for your support. I will check and make correction as per your suggestions.

0 anjana salgaonkar over 5 years ago in reply to anjana salgaonkar

Expert 1460 points

Hello roberto,
please confirm , The input specifications I have considering in excell sheet attchaed on 27th June are as correct. i mean those considered by you while designing PMP8740 design. if not then please correct.

0 Roberto S. over 5 years ago in reply to anjana salgaonkar

TI__Expert 5225 points

Hello Anjana,
The input specifications are correct. As far as I know the output voltage, taken into account in the excel sheet, is only "Vout" (Typical value) and not Vout_min (Minimum value) nor Vout_max (Maximum value). So you should put your actual output voltage in the cell "Typical".
Thanks,
Roberto

0 anjana salgaonkar over 5 years ago in reply to Roberto S.

Expert 1460 points

UCC28950 _PMP8740_ti.xlsxHello Roberto ,

1)In attached excel sheet power rating of DcDc converter considered by you for PMP8740 design is correct? because I am not able to get IPS=60.3 A as you mentioned in reply for how to calculate R98.

2) I am still not able to get value of Ls inductor =300uH as used in PMP8740 design. As per your suggestion I considered 15% load for calculating Ls, but still not get around 300uH value. Please help.

3) Also please explain output capacitor selection.

Thanks,

Anjana

0 anjana salgaonkar over 5 years ago in reply to anjana salgaonkar

Expert 1460 points

Hello Roberto ,
how do you select Rs in current sense network and Rsum in slope compensation network?
Anjana

0 Roberto S. over 5 years ago in reply to anjana salgaonkar

TI__Expert 5225 points

Hello Anjana,

Here are the answers about your questions:

1) The excel sheet you attached is correct. You get a different IPS number because your output nominal voltage is lower than my case.

2) Please consider that the 300uH value is placed in the BOM coming from a placeholder (in order to route the board layout). The right value is described in the Power Point Presentation file and also in the white paper, which is 10uH. In those documents is explained how it is built. Therefore I suggest you use also 10uH in your design and in the excel sheet.

3) I already replied to this question few answers ago, see:

Please let me know if that answers all your questions.

Regards,

Roberto

0 Roberto S. over 5 years ago in reply to anjana salgaonkar

TI__Expert 5225 points

Hello Anjana,
Rs and Rsum are calculated respectively on cells C113 and C224 of the excel sheet.
From more information, the application note or datasheet give a better explanation on the process of calculation.
Regards,
Roberto

0 anjana salgaonkar over 5 years ago in reply to Roberto S.

Expert 1460 points

Dear Roberto,

Attached is the excel sheet in which I am trying to match values selected in PMP8740. This Excel sheet is not as per my design specifications. I am trying to consider an input specifications which you have considered while designing PMP8740. Once this sheet get matched with PMP8740 design. I will use it by putting my inputs. So please check and tell me, inputs i am considering in the attached sheet are as same as those you have considered for PMP8740 design. Beacuse i am still not able to get values of Ls, Lout, Cout, in excel sheet as those used in PMP8740 design.

Anjana6675.UCC28950 _PMP8740.xlsx

0 anjana salgaonkar over 5 years ago in reply to anjana salgaonkar

Expert 1460 points

I just read your reply of 1july2018, 9.52Am. i will consider 10uH Ls value.

0 Roberto S. over 5 years ago in reply to anjana salgaonkar

TI__Expert 5225 points

Hello Anjana,

Please find attached the excel sheet about UCC28950, used to design the DC/DC stage of PMP8740.

It's a bit different from yours, but basically it's the same file with different parameters.

Best regards,

RobertoPMP8740 UCC28950 Excel Design Tool_SLUC222C.xls

0 anjana salgaonkar over 5 years ago in reply to Roberto S.

Expert 1460 points

dear Roberto ,

Please suggest alternate transformer for T2: AF4779.

Anjana

0 Roberto S. over 5 years ago in reply to anjana salgaonkar

TI__Expert 5225 points

Dear Anjana,
An alternate P/N of AF4779 is the P0584 from Pulse Engineering; the datasheet is downloadable under the link:
www.google.co.uk/url
Best regards,
Roberto

0 anjana salgaonkar over 5 years ago in reply to Roberto S.

Expert 1460 points

Thanks Roberto.

Anjana

0 anjana salgaonkar over 5 years ago in reply to anjana salgaonkar

Expert 1460 points

Dear Roberto,

Hope you are doing well.

I have procure P0584ANL transformer to replace AF4779 but footprint is not matching at all. is there any alternate part no which matches footprint and specifications of AF4779?

Anjana

0 Roberto S. over 5 years ago in reply to anjana salgaonkar

TI__Expert 5225 points

Dear Anjana,

I am fine, thanks....hope the same about you.

I checked out and, since AF4779 part number from Vitec is a custom, I didn't find a pin to pin replacement.

I am afraid you have to adapt your actual transformer to the layout, sorry.

Regards,

Roberto

0 anjana salgaonkar over 5 years ago in reply to Roberto S.

Expert 1460 points

Hello Roberto!

Hope you doing well!

What is function of Precharge signal? How it is used in PMP8740 design?

Anjana

0 anjana salgaonkar over 5 years ago in reply to anjana salgaonkar

Expert 1460 points

Also If my max output voltage is 60V then do I need to change Q32 in PMP8740 design? because Q32 is 160V Dual transistor And as i can see it receives signal from microcontroller (5V signal) and drives Q33, So I think i can use as it is in my design. Am I right? Please let me know selection procedure of Q32.

0 Roberto S. over 5 years ago in reply to anjana salgaonkar

TI__Expert 5225 points

Hello Anjana,

The digital signal "Pre-Charge" is enabling all FETs Q12, Q17, Q25...Q31: these form the "precharge" function, which means they are OFF at the beginning and let, by means of thermistors RT1 & RT2, charge the output capacitors C53...C56, C19 and C29 from the attached battery. Without this pre-charge function you get a spark on output terminals during battery connection. Furthermore the FETs Q12...Q31 provide reverse polarity protection, in case on output terminal you have negative voltage.

0 Roberto S. over 5 years ago in reply to anjana salgaonkar

TI__Expert 5225 points

This is correct! You can use the same Q32 dual-transistor I employed: you might want to increase the resistor R83 to limit the base current of Q33 and limit also the dissipation on R83.

0 anjana salgaonkar over 5 years ago in reply to Roberto S.

Expert 1460 points

Thanks a lot Roberto.

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